When I enable the Debug Mode for Lightning components in the Setup most of my Lightning components literally go crazy. They bombard me with cryptic erros like that and after a while my Chrome brings my CPU close to a halt.
Access Check Failed! AttributeSet.get(): attribute 'foo' of component 'markup://aura:iteration {24974:0}' is not visible to ...
I'm quite sure that this is not related to my tested clean JS code so I assume it's a bug.
Did you see those symptoms as well? Do you know of a Known (documented) issue?